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The talented Seattle-based indie writer/director Lynn Shelton relishes tossing her characters into awkward situations that test the boundaries of love and intimacy. Her 2009 film Humpday featured two long-time male friends, both straight, agonizing over the execution of a drunken dare to star in a gay porno together. In her latest film, Your Sister’s Sister, sibling love (and rivalry) is tested when two sisters end up in a vacation cabin with a man one (Emily Blunt) secretly loves and the other (Rosemarie DeWitt) has secretly slept with.

Plenty of filmmakers are drawn to the drama of discomfort—Todd Solondz has ushered a dazzling array of disastrous interactions onto the screen—but what sets Shelton apart is her interest in the way people can absorb acute discomfort and move on from it, even improving their lives because of it. Her characters tend to, at the beginning, have a sort of complacency of coolness; they believe themselves to be edgy and accepting, until they encounter a situation that calls for them to be truly edgy and accepting. I don’t want to consign this female director to a role that’s traditionally been more of a woman’s thing, but Shelton has a way of nurturing characters that make the occasionally wild evolutions of their relationships plausible. She helps them get there.

Iris (Blunt) a Seattle-based creative type (Shelton keeps details vague) used to date Jack’s (Mark Duplass, one of the stars of Humpday) brother, who died a year early under circumstances that aren’t exactly clear. One suspects, from Jack’s anger, that it might have been at his own hand. Iris and Jack are close—she can wrangle him when his bitterness is causing everyone around him to squirm—but technically, just friends. He’s between jobs and adrift, so Iris ships him off to her father’s vacation home in the San Juan Islands. It’s beautiful and serene, woodsy and overcast, the perfect setting to sit and think and come to terms with his misery. Instead he arrives late one evening to find Iris’ much more cynical older sister, Hannah (DeWitt), already settling in for her own pity party, having just broken up with her longtime girlfriend. The tequila comes out and the die is cast. Only it’s not. Without revealing any more plot detail, I’ll just say Your Sister’s Sister surprised me.

With only three characters, it’s like a round-robin of reactions. What do you do when you end up alone in a remote location with someone you’ve heard about but never met, a sort of endorsed stranger? Or when you realize that two people you thought belonged to you, each in their own way, have connected without you? The bare bones of the story came from Mark Duplass and his brother Jay, his regular collaborator and co-director of films like Cyrus and Jeff, Who Lives at Home, but Shelton fleshed it out and then the actors revised and improvised on set. There is a looseness to the dialogue that suits the mood of the story—each character gets his or her own bombshell (or two) to digest and has to figure out how to cope with it.

Your Sister’s Sister is an actor’s showcase. Duplass, who is everywhere these days—he is in three movies this month alone—is such a relaxed, naturalistic actor that he appears to be making no effort at all to play Jack, until you realize how long it takes you to figure out if you truly like and trust the character at all. That’s somewhat true of DeWitt’s Hannah as well. She initially presents as the slightly aloof, cool older sister, idolized by Iris—you see her weariness with and slight disdain for the way Iris tries to please everyone, which she, Hannah, doesn’t seem at all inclined to do—but layers of her vulnerability and need are exposed as the story unfolds. She does something atrocious and Hannah will likely prove a bone of contention for audiences, but DeWitt, who played the sister in Rachel Getting Married and had a memorable turn as Don Draper’s bohemian girlfriend in the first seasons of Mad Men, plays her with bitchy brio. She and Duplass have a long, loopy flirtatious scene together that’s so disastrously dumb it’s a delight to watch.

Playing against the sharp-tongued, precise type she established in The Devil Wears Prada and recently revisited in Salmon Fishing in the Yemen, Blunt has the harder task. The character walks a fine line—Iris could be seen as generous and sincere, or she could be a bit of a doormat. Blunt never seems more at home than when she’s crisply snapping out zingers, but there is nothing zingy about Iris. She’s even sweeter than the character Blunt played in April’s The Five Year Engagement. At times Blunt seems to be straining to fit in with Shelton’s naturalism but ultimately it turns out to be part and parcel of Iris, who adores both Hannah and Jack, but is thrown off by the vibe between them. When she and Hannah call each other nicknames (“Bean,” “Puppet”), there’s just the slightest undertone that they’re both trying too hard. It feels like that thing girls do when they are trying to claim each other, a kind of “let me braid your hair.”

I feel I have to be upfront about Your Sister’s Sister slightly ambiguous ending. That’s right on trend for the smart indie set (in the last year alone Like Crazy, Martha Marcy May Marlene, The Sound of My Voice and Another Earth have all left audiences hanging) but it also tends to tick off audiences who want to know, having invested their time and money in these characters, how they end up and what neat little box they can be dispatched to. Shelton cuts away at a crucial moment, opening the door for her audience to imagine various scenarios for the characters, Sliding Doors style. But no matter how poignantly undefined the specifics of their future are, Shelton has already shown them making a crucial decision together. It’s provocative, open-hearted and suggests that Hannah and her sister and her sister’s friend will be just fine.

Mary Pols has been reviewing film and occasionally books for TIME since 2009. She has degrees from Duke University, where she studied art history, and the University of California Berkeley’s School of Journalism. She is the author of a memoir about untraditional parenthood. It was made into a short-lived television comedy where everything turned out traditionally.

Accidentally on Purpose: The True Tale of a Happy Single Mother is available for nearly nothing at various online bookstores and from boxes in Pols' barn.